Anyone who wants to use the Fullscreen mode is already using it. If they believe people aren't using it because they don't know about it, then add a tooltip over the menu (which most users probably never open) and tell them about it.

Changing the user's settings without their approval is not in the spirit of "democratizing publishing". This is going to piss off a lot of people. And honestly, everyone will get over it, but a massive number of people are going to have their workday disrupted and have to search the interface to find a hidden button to return the menu and likely need to Google the solution. The end result is people learning that WP updates stink because they change their settings without asking and disrupt their workflow.